Re-enable -Wunknown-warning-option

Bug: http://b/68236396

This warning only needs to be enabled for frameworks/base/core/jni.

Test: Build the topic
Change-Id: I9f6aab2045f135d691696f9fd461c895a5aadb38

cc/config/clang.go

@@ -100,9 +100,6 @@ func init() {
 		// http://b/68236239 Allow 0/NULL instead of using nullptr everywhere.
 		"-Wno-zero-as-null-pointer-constant",
 
-		// http://b/68236396 Allow unknown warning options.
-		"-Wno-unknown-warning-option",
-
 		// http://b/36463318 Clang executes with an absolute path, so clang-provided
 		// headers are now absolute.
 		"-fdebug-prefix-map=$$PWD/=",